This is an advertisement, and a poor one at that. I doubt most folks in the HN crowd would consider being owned by Automattic as a benefit, given their recent treatment of the WordPress community.

It's not really clear where you download the said grammar checker or if it's open source. Looks more like a puff blog piece than something actionable.

Their "click here for the thing" is broken in the dumbest way

The goods: https://github.com/Automattic/harper (Apache 2)

Also, 'grammar' in this context is English grammar, not parsers, and no other languages